The term mast truck refers to an industrial truck or forklift mobile on at least three wheels, comprising a mast structure which consists of a fixed mast assembly and a lift carrier. It may further include movable intermediate carriers for increasing the lifting height of a truck or forklift. Thus, there will be two or more stages to the truck. The truck mast connects in a fixed or articulated fashion to the body of a truck as designated in standard EN 1726-1 or EN 1726-2, e.g. FIG. 1. In the mast, such stages or sections are movable relative to each other under the propulsion of pistons in hydraulic separate cylinders, from which the drive and power transmits either directly or over chains or cables to a load-lifting carrier. The mast and its stages constitute a telescopic guide system, wherein the stress is transmitted from the load-lifting carrier over guide rollers to the body structure of a truck. A mast truck is used, among others, for carrying containers, boxes or other such articles e.g. onto storage racks or vice versa.
The mast and carriers of a truck are typically assembled by substantially vertically set sections, such as I-beams, which are connected to each other with cross-members, and said mast and carriers being set in contact with each other to enable the same to travel vertically relative to each other through the intermediary of guide wheel bearings. The outermost intermediate carrier is movable through the intermediary of guide wheel bearings along the sections of a fixed mast assembly in vertical direction. The inner intermediate carriers are respectively movable relative to the outer intermediate carriers and the lift carrier relative to the innermost intermediate carrier. This results in a telescopic structure for lifting goods. The telescopic structure is typically operated by means of hydraulic cylinders.
The intended use of a mast truck entails that the truck should provide as good a visibility as possible in every direction, but it is particularly important to have a field of vision as extensive and unobstructed as possible in the truck driving direction. Further requirements for the truck include a compact size and agility as operation often takes place in cramped storage facilities. The construction used in prior art mast trucks diminishes the driver's visual field in the most important observation direction because of a broad blind area created by a mast structure and lifting cylinders associated therewith. In these currently available mast trucks, the hydraulic cylinders comprise a separate cylinder liner construction and a piston construction and are preferably mounted either alongside or behind the mast construction.
However, the publication DE 32 00 287 A1 discloses a prior known construction, wherein a U-section beam is integrally fitted with a tubular cylinder assembly. It is suggested that the tubular assembly be connected either to the end of a U-section or in the middle of the web on the side facing away from the legs. Manufacturing this prior known structure is nevertheless laborious and expensive.
The solution disclosed in DE 32 00 287 A1 is implementable in practice, e.g. by welding a cylinder-forming tubular structure to a U-section beam. An integral structure manufactured this way will be expensive and the manufacturing requires a multitude of operations demanding accuracy and special skills. Other manufacturing techniques for producing an integral beam as set forth in the cited publication include e.g. casting or machining. However, none of the above manufacturing methods is an economically sound way of making an integral beam. Furthermore, if the cylinder structure is attached to the end of a U-section, the construction will be very long as viewed in the traveling direction of a truck, which in turn leads to poorer handling characteristics for the truck. The most preferred way of manufacturing an integral beam would be by hot extrusion or cold drawing.
A hot extrusible apertured profile is manufactured according to the following operations:                drilling the initial blank, having a diameter of e.g. 150 mm and a length of 600 mm, for a hole with a diameter of e.g. 50 mm        heating the initial blank to red heat        pushing an auxiliary tool through the hole in the initial blank (rod diameter 50 mm)        pushing the initial blank, along with the auxiliary tool, through a profile form.        
If the hole is not in the centre of mass or in the immediate vicinity thereof, the auxiliary tool shall bend and break in the final operation. The same problem is encountered in the process of manufacturing a beam by cold drawing. Due to this requirement introduced by the manufacturing method, the manufacture of beams as disclosed in DE 32 00 287 A1 would have to be performed by adding “counterweight” for placing the centre of mass at the centre of the cylinder hole or in the vicinity thereof. Consequently, the integral beam will be heavy, its production consuming a lot of steel and its external dimensions increasing. On the other hand, the dimensionally increased mast structure obstructs too much of the visual field or has a negative effect on agility of the machine.